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Gorillas in zoos become agitated and anxious when large numbers of people approach their exhibit, new research suggests.

The research, published in the September issue of the journal Applied Animal Behaviour Science, is the first analysis on the influence of visitors on the behaviour and welfare of zoo gorillas.

"We noticed more behaviours suggestive of relaxation, such as increased resting, during low visitor density, and more behaviours suggestive of agitation, such as repetitive rocking, group-directed aggression and self-grooming during high visitor density," says the study's author, Dr Deborah Wells.

The gorilla group includes both wild-born and captive-born males and females of different ages.

The gorillas were observed for four hours a day on 20 busy days, when the average number of visitors was around 1288.

The gorillas were also observed on 20 quiet days, usually on weekdays when an average of six people visited the zoo.

During these periods, Wells documented known gorilla behaviours, such as standing, sitting, resting, grooming, aggression, playing, walking, running, climbing, socialising and banging on the viewing window.

She also recorded abnormal behaviour, like repetitive teeth clenching, body rocking and spinning.

Busy, busy, busy

Visitors seemed to have no effect on basic behaviours, such as standing, walking and socialising.

But when there were few visitors the gorillas tended to relax and rest more.

When the zoo was crowded, the gorillas often banged on the viewing window, exhibited all of the abnormal behaviours, repeatedly groomed themselves and fought more often.

Wells says visitors can affect all primates, not just gorillas. Since it is hard to know when zoos will be busy or not, she says factors other than visitor group size could lessen the possible human impact.

"Primates are more likely to be excited by zoo visitors if they appear in large numbers, display erratic behaviours, elicit high levels of noise or deviate greatly from the norm in terms of their physical appearance, for example height, clothing," says Wells.

She adds that some zoos even play classical music, which seems to soothe the animals and drown out visitor noise.

"[But] not all animals are bothered by crowds or noise. Some species and individuals [appear to be] ambivalent, [while] some appear to enjoy the presence of visitors," he says.

Challis says zoos should be sensitive to the requirements of animals, and that exhibits can be designed to benefit both animals and humans.

"Where it is necessary and relevant, efforts such as the maintenance of species in relevant social groups, off-show areas for retreat, restricted areas of viewing for visitors, sympathetic enclosure design with a knowledge of the species' biology in mind, use of closed-circuit television, or something similar, to show animals even if [they are] off-show, are all relevant," he says.

After the study, the zoo installed camouflage netting to the viewing area of the gorilla enclosure, which seemed to help reduce some of the abnormal behaviours and infighting.